The present invention relates to a spot welding machine for welding a plurality of plate materials or workpieces.
Spot welding is employed in many applications such as production line systems for automobile industries etc. since it allows thin metal plate materials (plate workpieces) to be efficiently welded in large quantity.
In such production line systems, there are popularly employed portable spot welding machines which are compact in size and light in weight and therefore can easily be moved to a desired welding spot. The portable spot welding machines, in general, comprise a welding gun, a secondary conductor, a welding transformer and a control device, and there are presently known two types of the portable spot welding machine, one being the type in which only a welding gun is moved to a welding spot, the other being the type in which a transformer gun comprising an integral combination of a welding gun and a transformer is moved to a welding spot.
The above-mentioned welding gun is typically constructed of a pressure source and pressing mechanism. As the pressure source, there are known an pneumatic type and an air hydraulic type, and as the pressing mechanism, there are known a C-shaped arm (press) type and a X-shaped arm (locker arm) type. Other conventionally known welding guns of this type are shown in, for example, U.S. Pat. Nos. 4,542,277 and 4,970,361, and U.K. Patent Application Laid-open Publication Nos. 2,211,128A, 2,211,129A and 2,216,298A.
In the welding guns of the C-shaped and X-shaped arm types, arms are caused to open or close by a pressing force supplied from the pressure source to thereby hold plate workpieces between tip electrodes, and the pressing force is applied to the workpieces between the tip electrodes. Then, welding current is applied to the electrodes to perform a spot welding operation.
But, because the arms are opened and closed only by the pressing force from the pressure source in order to control a distance between the electrodes (hereinafter referred to also as "interelectrode distance"), the prior art welding guns can set the interelectrode distance to only two extreme states, one being such a state where the arms are widely opened, i.e., the interelectrode distance is the maximum, the other being such a state where the arms are closed, i.e., the interelectrode distance is the minimum. The closed state of the arms means that plate workpieces are undergoing a spot welding operation while being held between the electrodes, and the opened state of the arms means that the welding spot is being moved with the electrodes being opened to the maximum degree. The prior art welding guns spot-weld the plate workpieces at their plural spots by repeating such opening and closing movements of the arms.
However, the prior art welding guns are not satisfactory in that a necessary operation time is very long due to the repeated opening and closing movements of the arms. Namely, after welding of the workpieces at one spot has been been completed, it is necessary to open the arms before the gun moves to a next welding spot. In other words, it is necessary to wait for a considerably long time until the arms are widely opened and hence the interelectrode distance reaches the maximum value. Therefore, the welding gun could not promptly move to the next welding spot, which was quite a waste of time.
Further, with the prior art welding guns, because the arms move a long distance from the opened state where the interelectrode distance is the maximum over to the closed state in order to hold the plate workpieces, a great impact tends to be given to the tip electrodes and plate workpieces when the electrodes come into contact with the workpieces. Such a contact impact causes the tip electrodes to wear in a very short period of time or causes damages like holes to the workpieces if they are thin.
Moreover, with the prior art welding guns, it is not possible to automatically detect such an unusual condition that the tip electrodes themselves remain attached or welded to the workpieces even after the arms are opened. Therefore, whether or not the tip electrodes are in the predetermined opened state must be directly checked with human eyes each time it is necessary to do so.